Skip to main content

World session

crowdy::session::WorldSession is the SDK-managed game-state layer — the C++ analog of CrowdyJS's World Stores. It moves the client-side bookkeeping every game otherwise hand-writes (actor registries, send loops, chunk caches, chat rings, host polling) into the SDK as typed, queryable stores driven by one replication connection and one tick.

session::WorldSessionConfig sc;
sc.appId = appId;
session::WorldSession world(conn, &game, sc);
world.join({0, 0, 0}, initialStateBytes);

// Game loop:
world.tick();  // polls the connection, drives the send loop, reaps, writes back

Everything is single-threaded by design: tick() polls the connection (dispatching notifications into the stores) on the calling thread — the same thread that reads them — so reads are plain snapshots with no locking.

Your actor: self()​

LocalActorStore drives your actor's presence:

  • Send-on-change at a fixed rate (default 5 Hz): setState(bytes) only marks the state dirty; unchanged states are deduped.
  • Periodic keyframes (default every 3 s): a full send goes out even when nothing changed, keeping presence fresh and repairing lost packets.
  • Idle heartbeats (default every 2 s): while unchanged, a cheap sendHeartbeat replaces the full update so presence never lapses.
  • Chunk moves send immediately: moveTo(chunk) does not wait for the next send slot — crossing a chunk boundary should never lag.
  • Acks: your own updates echo back from the server; lastAck() exposes the last applied echo (sequence, server time, state) for reconciliation.

State payloads are opaque bytes. For typed states, PodCodec<T> maps a trivially-copyable packed struct to the wire payload (the struct layout is the wire layout), and UnrealPose ships as a ready-made 88-byte layout interoperable with the Unreal SDK's pose format.

Everyone else: actors() and lanes​

RemoteActorStore is the registry of everyone you can see, fed from actor notifications with your own echoes filtered out:

  • Sample history per actor (newest-first, configurable depth) with server epoch and receive time — exactly what interpolation needs.
  • Staleness reaping: actors unseen for staleAfterMs (default 12 s) are reaped on tick, firing onLeave. onJoin / onUpdate fire as actors appear and move.
  • Lanes: named, filtered sub-registries so different actor kinds (players vs mobs, discriminated by a payload tag byte, for example) are decoded once and read separately.

The world: chunks()​

ChunkStore is a chunk/voxel cache combining the durable GraphQL store with the realtime stream:

  • Hydrate: ensureAround(center, distance) bulk-loads every stored chunk in range in one GraphQL round trip; pruneBeyond evicts far chunks (persisting dirty ones first).
  • Realtime merge: inbound voxel notifications are applied to the cache automatically.
  • Optimistic edits: setVoxel applies locally, replicates over UDP, and marks the chunk for durable write-back.
  • Worldgen write-back: chunks the server has never stored can be generated client-side (seed / insertGenerated) and persisted so the world stays identical for everyone — write-back is throttled (default one chunk per 700 ms) and flush() forces it.
  • onChunkChanged observes both realtime and local changes.

Events, messages, errors​

  • events() (EventRouter) routes client/server event notifications to per-eventType handlers and retains the last event per type. Send typed events with events().send(...).
  • channelInbox() / directInbox() (Inbox) retain channel messages and single-actor direct messages: drain oldest-first, peek without consuming, filter by channel, or observe with onMessage. Sending goes through the same objects (channel publish; fire-and-forget direct message).
  • errors() (ErrorStore) correlates server error frames with what you sent: the sequence-numbered error is attributed to the send kind (actor update, voxel update, text, …) that used that sequence, with recent(), lastFor(kind), and an onError callback.

Host tracking​

The session heartbeats host eligibility over GraphQL on an interval (default 3 s) and caches the election result: amIHost(), hostUserId(), and onHostChanged fire from tick when the elected host changes. The heartbeat is a blocking HTTP call on the tick thread — disable it (hostHeartbeatIntervalMs = 0) and run your own if that is unacceptable.

Durable stores​

Alongside the realtime session, thin caches wrap the durable GraphQL surfaces (durable and realtime payloads rarely share a layout, so these carry their own bytes):

  • SaveStateStore — the per-user, per-app save blob: explicit load() / save() with the bytes held locally between round trips, plus a byte-range patch.
  • AvatarStateStore — one avatar's identity-level and per-app state blobs, loaded and saved explicitly.
  • Actor-UUID persistence — your actor uuid should survive restarts so other players' registries treat you as the same actor. FileUuidStore persists it to a file (the native analog of CrowdyJS's localStorage-backed store); implement IUuidStore for your own storage.

Game-model state: ContainerMirror​

Game-model changes are pull-based on this platform (there is no model subscription); functions declare notify effects and clients re-read. This is the notify-to-pull pattern (see Model-driven notifications). ContainerMirror is the client half: watch the containers you care about, bind the mirror to a notification channel, and every watched container re-pulls its snapshot when that channel pings — you render straight from the cache. onChange fires with the container id, revision, and parsed properties whenever a snapshot actually changed.